Comment # 7 on bug 1091816 from Wolfgang Bauer

(In reply to Freek de Kruijf from comment #6)
> After setting a keyboard layout using systemsettings5 and after a reboot and
> choosing Plasma Wayland, I again have problems entering the mentioned
> characters. Checking the layout, adding a standard US keyboard also, there
> are still problems using the keys ' " and ^, in the setting with US keyboard
> with dead keys. Changing to the normal US keyboard layout makes it possible
> to enter these characters.
> In the setting with dead keys and when pressing the accent-key or the
> quote-key the popup which also shows when <Alt>+<F2> is pressed, pops up.

This should probably better be reported upstream though, IMHO.